1. Field of the Invention
The present disclosure relates to a method for driving a display panel, and more particularly, to a pixel driving method and apparatus for an organic light emitting device (OLED). Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for preventing a non-uniform brightness due to different levels of a common voltage at different positions within a display panel, and for preventing a flicker phenomenon due to a short data voltage emission period in a large display panel.
2. Description of the Related Art
Generally, an organic light emitting device (OLED) is a plane-type light emitting device. In an OLED, an organic light emitting layer is disposed between two electrodes facing each other so that when a voltage is applied between the two electrodes, electrons injected from one electrode are combined with holes injected from another electrode in the organic light emitting layer. As a result of the combination, molecules in the light emitting layer are excited such that light is emitted. Presently, the OLED is seen as the next generation of display apparatus due to its excellent viewing characteristics, light weight, thin thickness, and low voltage driving. The OLED is classified as either an Active-Matrix type OLED or a Passive-Matrix type OLED according to whether a switching device is provided in each of the unit pixels of a display panel.
FIG. 1A is a block diagram showing an OLED in accordance with the related art. As shown in FIG. 1A, the related art OLED includes a scan driving unit 10 for sequentially outputting scan signals to drive scan lines S1-Sn on a display panel 30 under control of a signal controller (not shown); a data driving unit 20 for outputting data voltages to data lines D1-Dm on the display panel 30; and a display panel 30 having a plurality of pixels PXs at intersections between the scan lines S1-Sn and the data lines D1-Dm. The pixels of the active-matrix type OLED are driven by one of voltage writing, current writing and digital writing.
FIG. 1B is a circuit for driving pixels PXs on the display panel 30 of FIG. 1A. As shown in FIG. 1B, the pixel circuit includes a switching transistor T11 transmitting data voltages DATA supplied through the data lines D to a storage capacitor C11 by being driven by the scan signals SCAN supplied through the scan lines S; the storage capacitor C11 for being charged to the data voltage DATA is also connected between a gate terminal of a driving transistor T12 and a lower power supply voltage terminal Vss; a driving transistor T12 supplies a driving current to an organic light emitting diode OLED 11 having a brightness corresponding to the driving current by having an anode connected to an upper power supply ELVDD voltage terminal and having a cathode connected to a drain of the driving transistor T12. The driving current corresponds to the data voltage DATA charged onto the storage capacitor C11. The transistors T11 and T12 are implemented as N-channel type thin film transistors (TFTs).
FIG. 2 is a waveform of FIGS. 1A and 1B. FIG. 3 is a schematic view showing an arrangement structure of power supply voltage supply lines on a display panel. The operation of the related art circuit for driving pixels shown in FIGS. 1A and 1B will be explained with reference to FIGS. 2 and 3.
In each frame period as shown in FIG. 2, positive scan signals Scan [1]-Scan [N] are sequentially supplied from the scan driving unit 10 to the scan lines S1-Sn on the display panel 30, thereby driving the pixels PXs on a corresponding scan line (horizontal line). As also shown in FIG. 2, an upper power supply voltage ELVDD of a certain level (i.e., 15V) is continuously supplied to the anode of the OLED 11 for one frame period. FIG. 1B is an exemplary view showing just one of a plurality of pixels (including a driving circuit) connected to an optional scan line.
The switching transistor T11 is turned ON by a corresponding scan signal among the scan signals Scan [1]-Scan [N]. The data voltage DATA supplied from the data driving unit 20 through a corresponding data line among the data lines D1-Dm charges the storage capacitor C11 through the switching transistor T11, and is maintained for a data voltage emission period. The driving transistor T12 is turned ON by the data voltage DATA charged onto the storage capacitor C11, and a certain amount of driving current corresponding to the data voltage DATA flows through the OLED 11. Accordingly, the organic light emitting diode OLED 11 emits light with a brightness corresponding to the data voltage DATA.
The driving current IOLED flowing in the OLED 11 is expressed as the following equation 1.
                              I          OLED                =                              1            2                    ·                      W            L                    ·                      C            SINx                    ·                                    {                                                V                  DATA                                -                Vss                -                                  V                  TH                                            }                        2                                              [                  Equation          ⁢                                          ⁢          1                ]            Here, “L” denotes a channel length of the driving transistor T12, the “W” denotes a channel width of the driving transistor T12, the “CSINx” is a capacitor component of a gate insulator, the “VTH” denotes a threshold voltage, and the “VDATA” is a data voltage charged onto the storage capacitor C11.
As shown in FIG. 3, a lower power supply voltage Vss supply line 32 is arrayed on an array portion 31 with a mesh structure so as to minimize a resistance. On each outer periphery of the array portion 31 and the display panel 30, other lower power supply voltage supply lines 33 and 34 having a wider width are arrayed, thereby smoothly supplying the lower power supply voltage Vss.
In a data voltage programming period, when data voltages are being charged onto the storage capacitors C11 of the pixels PXs inside the display panel 30, about 1 μA of current flows through the OLED 11 and the driving transistor T12. The current flows to the lower power supply voltage supply lines 33 and 34 through the lower power supply voltage supply line 32. Accordingly, the current flowing in the display panel 30 has a total amount corresponding to several tens of mA, and thus a potential on the lower power supply voltage supply line 32 is increased. The increased lower power supply voltage Vss′ is expressed as the following equation 2.Vss′=Vss+IOLED·Rline   [Equation 2]The driving current IOLED of the OLED 11, and the resistance Rline of the lower power supply voltage supply line 32 have different values depending on position inside of the display panel 30.
As the potential on the lower power supply voltage supply line 32 is increased, a driving voltage of the driving transistor T12 inside the pixel is lowered, thereby lowering a brightness of the OLED 11. As the lower power supply voltage Vss changes to Vss′, the driving current IOLED of the OLED 11 is lowered, which is expressed as the following equation 3.
                              I          OLED                =                                            1              2                        ·                          W              L                        ·                          C              SINx                        ·                                          {                                                      V                    DATA                                    -                                      Vss                    ′                                    -                                      V                    TH                                                  }                            2                                ≤                                    1              2                        ·                          W              L                        ·                          C              SINx                        ·                                          {                                                      V                    DATA                                    -                  Vss                  -                                      V                    TH                                                  }                            2                                                          [                  Equation          ⁢                                          ⁢          3                ]            
The potential on the lower power supply voltage supply line 32 is increased at the time of programming the data voltages due to the organic light emitting diode (OLED) of each pixel, the lower power supply voltage supply line 32 having a mesh structure, and the current flowing the lower power supply voltage supply line 32. Accordingly, the driving voltage of the driving transistor inside the pixel is lowered, thereby lowering brightness of the organic light emitting diode depending on the location of the pixel in the mesh. Since the brightness can be lowered at respective pixels by different levels, a non-uniform brightness can result in the overall display panel.